On-die system and method for controlling termination impedance of memory device data bus terminals

ABSTRACT

A system for controlling the termination impedance of memory device data bus terminals is fabricated on the same die as the memory device. The system includes a termination resistor connected to each data bus terminal, which is connected in parallel with several transistors that are selectively turned on to adjust the termination impedance. The transistors are controlled by a circuit that determines the resistance of the termination resistor and turns on the correct number of transistor to properly set the termination impedance. In one example, the resistance of the termination resistor is determined by directly measuring a resistor of the same type as the termination resistor. In another example, the resistance of the termination resistor is determined indirectly by measuring parameters that affect the resistance of the termination resistor. In either case, the system can maintain the termination impedance of the data bus terminals constant despite changes in the termination resistor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/643,835, filed Dec. 21, 2009, which is a continuation of U.S. patentapplication Ser. No. 11/804,176, filed May 16, 2007, U.S. Pat. No.7,646,213. These applications are incorporated by reference herein intheir entirety and for all purposes.

TECHNICAL FIELD

This invention relates to memory devices, and more specifically, tocontrollably adjusting the termination impedance of data bus terminalsthat are used to couple write data to and read data from the memorydata.

BACKGROUND OF THE INVENTION

Memory devices are typically assembled into memory modules that are usedin a computer system. These memory modules typically include singlein-line memory modules (SIMMs) having memory devices on one side of thememory module, and dual in-line memory modules (DIMMs) having memorydevices on both sides of the memory module. The memory devices of amemory module are accessed in groups. Each of the groups are commonlyreferred to as “ranks,” with single-sided DIMMs typically having onerank of memory devices and double-sided DIMMs having two ranks of memorydevices, one rank on either side of the memory module.

Each of the memory devices of a memory module receives a set of signals,which is generated by a memory controller. These signals include commandsignals for specifying the type of access of a memory device, such as aread or a write, address signals specifying the location in the memorydevice being accessed, and write data signals corresponding to data thatare to be stored in the memory device. The memory device can alsotransmits to the memory controller read data signals corresponding todata that have been stored in the memory device.

As the operating speed of memory devices continues to increase, timingmargins for the various signals related to memory device operationbecome more critical, particularly for data signals, which are generallytransmitted and received at a higher rate than command and addresssignals. Subtle variations in signal timing and operating conditions cannegatively impact memory device performance. Consequently, it isdesirable to improve timing margins without sacrificing performance,where possible.

One factor that can adversely affect timing margins is reflection ofsignals in conductors through which data signals are coupled. Write datasignals and read data signals are typically coupled through a data busthat is coupled to several memory devices. As is well known to oneskilled in the art, the conductors of the data bus are transmissionlines, which have a characteristic impedance. If the impedance of amemory device data bus terminal is not matched to the characteristicimpedance of the data bus conductors, write data signals transmitted tothe memory device will be partially reflected from the data busterminals. Similarly, read data signals transmitted to a memorycontroller will be partially reflected from the data bus terminals ofthe memory controller if the impedance of the data bus terminals doesnot match the characteristic impedance of the data bus conductors. Thesereflected read and write data signals can remain present on the data busas subsequent data signals are coupled through the data bus, and theycan alter in spurious manner the timing of transitions of thesesubsequent data signals or the amplitude of these subsequent datasignals. The result is a reduction in the timing margins of the memorydevice.

One approach to improving memory device timing margins is the use ofon-die termination (“ODT”) circuits for data bus terminals to which datainput/output buffers are connected. The ODT circuits provide resistiveterminations that are approximately matched to the characteristicimpedance of the data bus conductors to reduce reflections and therebyimprove timing margins of the memory device.

The ODT circuits used in a conventional memory device are typicallydisabled when the memory device is not receiving write data, and theyare enabled when the memory device is receiving write data. When the ODTcircuit is disabled, the impedances of its associated data bus terminalsare very high to simulate an “open circuit” condition in which thememory device is not connected to the data bus, under this condition,the data bus terminals do not substantially reflect data signals. Atypical ODT circuit 10 is shown in FIG. 1. The ODT circuit 10 includes aseries combination of a first termination resistor 12, which isconnected to a supply voltage V_(CC), a PMOS switching transistor 14, anNMOS switching transistor 16 and a second termination resistor 18, whichis connected to ground. The data bus terminal DQ is connected betweenthe transistors 14, 16. The transistor 14 is selectively turned ON by anactive high enable signal En, and the transistor 16 is turned ON by itscomplement, which is generated by an inverter 20.

In operation, the ODT circuit 10 is disabled by an inactive low Ensignal to turn OFF the transistors 14, 16. The DQ terminals are thus“tri-stated” at a high impedance. When the transistors 14, 16 are turnedON by the active high En signal, the resistors 12, 18 essentially form avoltage divider to set the impedance and bias voltage of the DQ terminalto predetermined values. The resistance of the resistors 12, 18 aregenerally equal to each other so that the DQ terminal has an impedanceof one-half the resistance of the resistors 12,18, and it is biased to avoltage of one-half the supply voltage V_(CC).

The ODT circuit 10 shown in FIG. 1 can markedly reduce the signalreflections from the DQ terminal. However, its performance in thisregard is less than optimum because the resistance of the resistorsgenerally cannot be precisely controlled. As is well-known in the art,the resistors 12, 18 are generally fabricated from a polysiliconmaterial. Presently existing semiconductor fabrication techniques do notallow the resistance of polysilicon resistors to be precisely controlledbecause of process variations. Even if the resistors 12, 18 could befabricated with the correct resistances, the resistances would changewith time as well as other factors such as temperature changes andsupply voltage variations. As a result, the DQ terminals of conventionalmemory devices using the ODT circuit 10 still cause considerablereflections.

One approach that has been used to deal with the inability to fabricatepolysilicon resistors with precisely controlled resistances is an ODTcircuit 30 as shown in FIG. 2. The ODT circuit 30 uses many of the samecomponents that are used in the ODT circuit 10 of FIG. 1. Therefore, inthe interest of brevity, these components have been provided with thesame reference numerals, and an explanation of their characteristics andfunctions will not be repeated. The ODT circuit 30 differs from the ODTcircuit 10 shown in FIG. 1 by connecting a plurality of PMOS transistors34 a,b . . . n in parallel with the first termination resistor 12.Similarly, a plurality of NMOS transistors 36 a,b . . . n are connectedin parallel with the second termination resistor 18. The transistors 34a,b . . . n and 36 a,b . . . n are selectively turned ON by signals froma fuse bank 38.

In operation, the termination resistors 12, 18 are intentionallyfabricated with resistances that are higher than target resistances.During wafer test, the impedance at the DQ terminal is measured todetermine the resistances of the resistors 12, 18. A conventionalprogramming device (not shown) is then used to program a pattern offuses or anti-fuses in the fuse bank 38 to provide signals thatselectively turn ON the transistors 34 a,b . . . n, 36 a,b . . . n.Turning ON the transistors 34 a,b . . . n, 36 a,b . . . n lowers theresistance of the parallel combination of the resistor 12 and thetransistors 34 a,b . . . n and the resistance of the parallelcombination of the resistor 18 and the transistors 36 a,b . . . n. Thedegree to which the resistances are lowered depends on the number oftransistors 34 a,b . . . n, 36 a,b . . . n that are turned ON. Thenumber of transistors 34 a,b . . . n, 36 a,b . . . n that are turned ONcorresponds to the number of fuses or anti-fuses programmed by theprogrammer. The programmer therefore programs the fuse bank 38 based onthe DQ impedance measurement to couple the correct number of transistors34 a,b . . . n, 36 a,b . . . n in parallel with the resistors 12, 18,respectively, to provide close to the target DQ impedance.

The ODT circuit 30 shown in FIG. 2 provides a substantial improvement inDQ terminal impedance control over the use of the ODT circuit 10 shownin FIG. 1. However, it still suffers from a number of shortcomings,which cause the DQ terminal to significantly reflect signals applied tothe DQ terminal. The primary limitation of the ODT circuit 30 resultsfrom changes in the resistances of the resistors 12, 18, as well aschanges in the ON impedance of the transistors 34 a,b . . . n, 36 a,b .. . n over time and as a function of temperature and voltage variations.Therefore, even if the ODT circuit 30 can be precisely programmed withthe correct DQ termination impedance during fabrication, the DQtermination impedance may not be correct after a memory devicecontaining the ODT circuit 30 has been placed in operation. It is notpossible to reprogram the fuse bank 38 to provide the correct DQtermination impedance because the fuse bank 38 must be programmed beforethe memory device containing the ODT circuit 30 has been packaged.Furthermore, a considerable time can be required during fabrication totest the resistance of the termination resistors 12, 18 and to thenprogram the fuse bank, which can unduly increase the fabrication costsof memory devices containing the ODT circuit 30.

There is therefore a need for an ODT circuit that does not requireexpensive and time consuming testing and programming during fabrication,that can be fabricated with the correct DQ termination impedance, andthat can adapt to changes in the ODT circuit with time as well astemperature, process and supply voltage variations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one example of a conventional ODTcircuit that is commonly used in memory devices.

FIG. 2 is a schematic diagram of another example of a conventional ODTcircuit that is commonly used in memory devices to address some of theshortfalls of the ODT circuit shown in FIG. 1.

FIG. 3 is a block diagram of an ODT system according to one example ofthe invention.

FIG. 4 is a block diagram of an ODT system according to another exampleof the invention.

FIG. 5 is a block diagram of an ODT system according to another exampleof the invention.

FIG. 6 is a block diagram of an ODT system according to still anotherexample of the invention.

FIG. 7 is a block diagram of a memory system using an ODT systemaccording to one example of the invention.

FIG. 8 is a block diagram of a computer system using the memory systemof FIG. 7.

DETAILED DESCRIPTION

An ODT system 50 according to one example of the invention is shown inFIG. 3. The ODT system 50 will generally be fabricated on the same chipas a memory device with which the ODT system 50 is used. The ODT system50 includes an ODT circuit 54, which may be identical to the ODT circuit30 shown in FIG. 2 except that the fuse bank 38 is omitted. Instead ofprogramming the ODT circuit 50 with signals from a fuse bank, the ODTcircuit 54 is programmed by signals from a termination resistancedetermining circuit 56. The termination resistance determining circuit56 determines the termination impedance provided by the terminationresistors 12, 18 (FIG. 2) without any of the transistors 34 a,b . . . n,36 a,b . . . n turned ON. As explained in greater detail below, thetermination resistance determining circuit 56 determines the terminationimpedance either directly, such as by measuring the resistance of apolysilicon resistor like those used in the ODT circuit 54, orindirectly by measuring various parameters that affect the resistance ofthe polysilicon resistor like used in the ODT circuit 54. Once thetermination resistance determining circuit 56 has determined theresistance of the polysilicon terminal resistors used in the ODT circuit54, it applies appropriate signals to the ODT circuit 54 to turn ON thecorrect number of transistors 34 a,b . . . n, 36 a,b . . . n so that theDQ termination impedance is substantially equal to the target impedance.

Although the ODT system 50 shown in FIG. 3 uses the terminationresistance determining circuit 56 to provide signals to a single ODTcircuit 54, it can instead provide signals to the single ODT circuits 54for all or a subset of all of the DQ terminals of the memory device.Also, if the termination impedance of other terminals of the memorydevice, such as a clock terminal, are to be controlled, the ODT systemmay be coupled to these terminals as well.

An example of an ODT system 60 that directly determines the DQtermination impedance provided by the termination resistors in the ODTcircuit 54 is shown in FIG. 4. A termination resistance determiningcircuit 64 used in the ODT system 60 includes a polysilicon terminationresistor 62 that is substantially the same as the polysilicontermination resistors 12, 18 used in the ODT circuit 54. The resistor 62is connected in series with a current source 64. The voltage V_(S) at anoutput node 66 is equal to the product of the current provided by thecurrent source 64 and the resistance of the resistor 62. Therefore, thevoltage V_(S) at an output node 66 is directly proportional to theresistance of the polysilicon resistor 66.

The voltage V_(S) is applied to an input of an analog-to-digitalconverter (“A/D/C”) 70, which provides a binary value corresponding tothe magnitude of the voltage V_(S). The binary value output from theA/D/C 70 thus provides an indication of the resistance of thepolysilicon resistor 62. The number of bits output by the A/D/C 70 willdepend upon the desired degree of precision at which the resistance ofthe resistor 62 is to be determined.

The binary value output from the A/D/C 70 is applied to a set of latches72 a,b . . . n, each of which stores a respective bit of the binaryvalue provided by the A/D/C 70. The binary value stored in the latches72 a,b . . . n is applied to a control generator 74, which generatesdigital activation signals for the transistors 34 a,b . . . c, 36 a,b .. . c in the ODT circuit 54. More specifically, the control generator 74applies digital activation signals to the gates of respective PMOStransistors 14 a,b . . . n and NMOS transistors 16 a,b . . . n in theODT circuit 54 to set the DQ termination impedance to the target value.

The latches 72 a,b . . . n may be either volatile or non-volatile. Ifthe latches 72 a,b . . . n are volatile, it is necessary for the ODTsystem 60 to measure the resistance of the resistor 62 each time poweris applied to the ODT system 60. If the latches 72 a,b . . . n arenon-volatile, the same resistance measurement can be continuously useduntil a new measurement is desired. In some examples of the ODT system60, a new measurement is made at periodic intervals. In other examplesof the ODT system 60, a new measurement is made only when it is manuallyrequested. In still other examples of the ODT system 60, a newmeasurement is made only when the performance of a memory devicecontaining the ODT system 60 appears to be suffering from excessivesignal reflections from the DQ terminals. Other criteria can also beused to determine when a new measurement should be made.

A more detailed example of an ODT system 76 that directly determines theDQ termination impedance provided by the termination resistors in theODT circuit 54 is shown in FIG. 5. The ODT system 76 uses many of thesame components that are used in the ODT system 60 of FIG. 4. Therefore,in the interest of brevity and clarity, these common components will beprovided with the same reference numerals, and an explanation of theirfunctions will not be repeated. Instead of using a current source 64,termination resistance determining circuit 56 in the ODT system 76 usesa series combination of a PMOS switching transistor 78 and resistor 80having a precisely controlled resistance. The transistor 78 is turned ONby an active low ADJ EN F signal. The resistor 80 may be a conventionalresistor known as an “Naa” resistor, which is essentially a long path ofN⁺ doping in a p-substrate. Using Naa resistors for all of thetermination resistors in the ODT circuits 54 would eliminate theimpedance matching problems discussed above. However, the large amountof surface area that Naa resistors use on a semiconductor die would makethe cost of this approach prohibitive. In contrast, the amount of diearea consumed by one or a few Naa resistors uses relatively little diearea, and thus does not have a significant adverse impact on the cost ofa memory device containing the ODT system 76.

When the transistor 78 is turned ON, the resistors 80, 62 form a voltagedivider so that the voltage V_(S) at an output node 82 is given by theformula:

V _(S)=(R ₆₂/(R ₆₂ +R ₈₀))*/V _(CCP)  [Equation 1]

where R₆₂ is the resistance of the polysilicon resistor 62, and R₈₀ isthe resistance of the Naa resistor 80. The voltage V_(S) thus provides ameasurement of the resistance of the polysilicon resistor 62, whichsubstantially matches the resistance of the polysilicon resistors 12, 18used in the ODT circuit 54. By making the resistance of the Naa resistor80 substantially larger than the resistance of the polysilicon resistor62, the resistor 80 essentially acts as a current source, and Equation 1can be approximated as:

V _(S)=(R ₆₆ /R ₇₀)N _(CCP)  [Equation 2].

Using Equation 2, the magnitude of the voltage V_(S) is directlyproportional to the resistance of the polysilicon resistor 62. As theresistance of the resistors 12, 18 change with time or as a function oftemperature or supply voltage variations, the voltage V_(S) will changeaccordingly so that the changed value of the resistance can bedetermined.

The voltage V_(S) is applied to the analog-to-digital converter(“A/D/C”) 70 that was used in the ODT system 60 of FIG. 4. The binaryvalue output from the A/D/C 70 is applied to a trim code register 84,which includes a set of latches 86 a,b . . . n, each of which stores arespective bit of the binary value. The trim code register 84 may alsoinclude a decode circuit 88 for converting the bits of the binary numberto digital activation signals for the transistors 34 a,b . . . c, 36 a,b. . . c in the ODT circuit 54. Therefore, the latches 86 a,b . . . n maystore either the bits of the binary value from the A/D/C 70 or digitalactivation signals derived from the binary value.

Regardless of whether the trim code register 84 stores bits of thebinary signal from the A/D/C 70, the digital activation signals, or someother intermediate signals, the trim code register 84 applies digitalactivation signals to the gates of respective PMOS transistors 14 a,b .. . n and NMOS transistors 16 a,b . . . n in the ODT circuit 54 to setthe DQ termination impedance to the target value. As with the latches 72a,b . . . n used in the ODT system 60 of FIG. 4, the latches 86 a,b . .. n used in the trim code register 84 may be either volatile ornon-volatile.

An example of an ODT system 90 that indirectly determines the DQtermination impedance provided by the termination resistors in the ODTcircuit 54 is shown in FIG. 6. The ODT system 90 includes a conventionaltemperature sensor 92, which determines the temperature of asemiconductor die in which the ODT system 90 is fabricated and providesa corresponding output signal. The ODT system 90 also includes an MOSsensor 94, which measures the ON resistance of the ODT enabletransistors 14, 16 in the ODT circuit 54 and provides a correspondingoutput signal. Finally, the ODT system 90 includes a supply voltagesensor 96, which provides an output signal indicative of the magnitudeof a supply voltage. As explained above, all of the conditions measuredby the sensors 92, 94, 96 are those that affect the resistance of thepolysilicon termination resistors 12, 18 used in the ODT circuit 54.Therefore, measuring the value of these conditions allows the resistanceof the polysilicon termination resistors 12, 18 to be indirectlydetermined.

The signals from the sensors 92, 94, 96 are applied to a control logiccircuit 98, which generates an output signal indicative of the DQtermination impedance without any of the ODT enable transistors 14, 16in the ODT circuit 54 turned ON. Basically, the control logic circuit 98simply implements a known mathematical formula specifying the resistanceof a polysilicon termination resistors 12, 18 as a function oftemperature and supply voltage, and then adds to that the ON resistanceof the ODT enable transistors 14, 16 as determined by the MOS sensor 94.

The control logic circuit 98 outputs a binary value of the DQtermination impedance to the trim code register 84, which operates inessentially the same manner as previously explained with reference toFIG. 5. The trim code register 84 then drives the ODT enable transistors14, 16 in the ODT circuit 54 in the same manner as also previouslyexplained with reference to FIGS. 4 and 5.

The ODT system according to various examples of the invention, includingthose previously described, may be used in a variety of electronicdevices. For example, the ODT system may be used in a memory devices,such as a synchronous dynamic random access memory (“SDRAM”) device 100shown in FIG. 7. The SDRAM device 100 includes a command decoder 104that controls the operation of the SDRAM 100 responsive to high-levelcommand signals received on a control bus 106. These high level commandsignals, which are typically generated by a memory controller (not shownin FIG. 7), are a clock enable signal CKE*, a clock signal CLK, a chipselect signal CS*, a write enable signal WE*, a row address strobesignal RAS*, a column address strobe signal CAS*, and a data mask signalDQM, in which the “*” designates the signal as active low. The commanddecoder 104 generates a sequence of command signals responsive to thehigh level command signals to carry out the function (e.g., a read or awrite) designated by each of the high level command signals. Thesecommand signals, and the manner in which they accomplish theirrespective functions, are conventional. Therefore, in the interest ofbrevity, a further explanation of these command signals will be omitted.

The command decoder 104 also includes a mode register 108 that can beprogrammed by a user to control the operating modes and operatingfeatures of the SDRAM 100. The mode register 108 is programmedresponsive to a load mode (“LDMD”) command, which is registeredresponsive to a predetermined combination of the command signals appliedto the command decoder 104 through the control bus 106. One of theoperating features that can be programmed into the mode register is thepreviously described on die termination (“ODT”) feature.

The SDRAM 100 includes an address register 112 that receives rowaddresses and column addresses through an address bus 114. A row addressis generally first received by the address register 112 and applied to arow address multiplexer 118. The row address multiplexer 118 couples therow address to a number of components associated with either of twomemory banks 120, 122 depending upon the state of a bank address bitforming part of the row address. Associated with each of the memorybanks 120, 122 is a respective row address latch 126, which stores therow address, and a row decoder 128, which decodes the row address andapplies corresponding row activation signals to one of the arrays 120 or122. The row address multiplexer 118 also couples row addresses to therow address latches 126 for the purpose of refreshing the memory cellsin the arrays 120, 122. The row addresses are generated for refreshpurposes by a refresh counter 130, which is controlled by a refreshcontroller 132.

After the row address has been applied to the address register 112 andstored in one of the row address latches 126, a column address isapplied to the address register 112. The address register 112 couplesthe column address to a column address latch 140. Depending on theoperating mode of the SDRAM 100, the column address is either coupledthrough a burst counter 142 to a column address buffer 144, or to theburst counter 142, which applies a sequence of column addresses to thecolumn address buffer 144 starting at the column address output by theaddress register 112. In either case, the column address buffer 144applies a column address to a column decoder 148.

Data to be read from one of the arrays 120, 122 is coupled to columncircuitry 150, 152 for one of the arrays 120, 122, respectively. Thedata is then coupled from the column circuitry 150, 152 through a dataoutput register 156 to data bus terminals 158. Data to be written to oneof the arrays 120, 122 are coupled from the data bus terminals 158 to adata input register 160. The write data are coupled from the data inputregister 160 to the column circuitry 150, 152 where they are transferredto one of the arrays 120, 122, respectively. A mask register 164selectively alters the flow of data into and out of the column circuitry150, 152, such as by selectively masking data to be read from the arrays120, 122.

The SDRAM 100 also includes an ODT system 168 coupled to the data busterminals 158 for selectively controlling the termination impedance ofthe data bus terminals 158. The ODT system 168 may be the ODT system 50shown in FIG. 3, the ODT system 60 shown in FIG. 4, the ODT system 76shown in FIG. 5, the ODT system 90 shown in FIG. 6, or some otherexample of an ODT system according to the invention. As explained above,the ODT system 168 precisely controls the termination impedance of thedata bus terminals 158 despite changes in the resistance of polysiliconor other termination resistors (not shown) used in an ODT circuit thatis part of the ODT system 168. Although the ODT system 168 is shown as aseparate component of the SDRAM 100, it will be understood that it maybe combined with or incorporated in other components. For example, theODT system 168 may be incorporated in the data input buffer 160.

The SDRAM 100 shown in FIG. 7 can be used in various electronic systems.For example, it may be used in a processor-based system, such as acomputer system 200 shown in FIG. 8. The computer system 200 includes aprocessor 202 for performing various computing functions, such asexecuting specific software to perform specific calculations or tasks.The processor 202 includes a processor bus 204 that normally includes anaddress bus, a control bus, and a data bus. In addition, the computersystem 200 includes one or more input devices 214, such as a keyboard ora mouse, coupled to the processor 202 to allow an operator to interfacewith the computer system 200. Typically, the computer system 200 alsoincludes one or more output devices 216 coupled to the processor 202,such output devices typically being a printer or a video terminal. Oneor more data storage devices 218 are also typically coupled to theprocessor 202 to allow the processor 202 to store data in or retrievedata from internal or external storage media (not shown). Examples oftypical storage devices 218 include hard and floppy disks, tapecassettes, and compact disk read-only memories (CD-ROMs). The processor202 is also typically coupled to cache memory 226, which is usuallystatic random access memory (“SRAM”), and to the SDRAM 100 through amemory controller 230. The memory controller 230 normally includes acontrol bus 236 and an address bus 238 that are coupled to the SDRAM100. A data bus 240 is coupled from the SDRAM 100 to the processor bus204 either directly (as shown), through the memory controller 230, or bysome other means.

Although the present invention has been described with reference to thedisclosed examples, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. Such modifications are well within the skillof those ordinarily skilled in the art. Accordingly, the invention isnot limited except as by the appended claims.

1. A termination circuit fabricated on a semiconductor die along with anelectronic device having an externally accessible terminal, thetermination circuit comprising: a programmable on-die termination(“ODT”) circuit coupled to the externally accessible terminal, theprogrammable ODT circuit providing the externally accessible terminalwith an impedance using the combination of a terminating resistor and acontrollable impedance element having an impedance that is controlled byat least one programming signal received by the programmable ODTcircuit; and a programming circuit operable to determine the approximateimpedance of the terminating resistor and to provide the at least oneprogramming signal to the ODT circuit so that the impedance provided bythe combination of the terminating resistor and the controllableimpedance element is substantially equal to a predetermined targetimpedance.